> Can someone here on hn with more in deepth knowelege about ZFS commenting on why it is superior to EXT4 for example for file storage? Does each dir handle more children for example?

I'll tell you why I use it: Built-in compression, and data checksums (so it can catch and potentially correct corruption). Both are extra useful on storage arrays.

> Last time I read here HN ZFS still seem have edge case bugs. Has it matured now?

The only significant bugs I've heard of in a long time are with encryption. Still not ideal, but not a show-stopper.

> Why don't distro such as debian etc just ship ZFS as the default instead of ext4?

(The following is an oversimplification; I'm not a lawyer, so take with grain of salt.) There is, or at least may be, a licensing conflict between ZFS under the CDDL and Linux under the GPLv2. Both good open source licenses, but it is at best unclear that it's legal to distribute binaries that mix code under the two licenses. This makes it at best really messy to distribute a Linux distro using ZFS. (The generally accepted solution is to compile on-device, since the problem only happens with binaries.)